This invention relates to a magnetic dry ink powder suitable for use in electrophotographic recording. More particularly, the invention relates to a magnetic developing powder which is pressure responsive such that it can be fixed as an imaging material to an image-bearing surface solely by the application of pressure.
Known developing powders, i.e., dry toner formulations used in electrophotographic recording processes are typically permanently affixed to a copy by means of heat. An exemplary powder is described in Nelson, U.S. Pat. No. 3,639,245 wherein the powder is described as being thermoplastic and heat-fusible. Such heat-fusible powders are fixed after image formation by elevating the temperature of the powder to its melting or softening point, thereby causing the particles to coalesce, flow together and adhere permanently to the substrate.
Although such dry heat-fusing developable powders have been widely used and have met with commercial success, there have been certain disadvantages inherent in the use thereof, such disadvantages relating to speed and efficiency of the fixing process, image quality, etc.
Accordingly, a number of patents have issued which disclose the use of dry pressure-fixable developing powders to overcome these aforementioned deficiencies. For example, see U.S. Pat. Nos. 3,965,022 and 3,775,326 which both relate to pressure-fixing powders.
Such powders are generally soft and thus susceptable to humidity and temperature stability problems. Furthermore, the softness of these powders can result in lower image quality due to background toning or scumming.
When the resultant imaged substrate is simply used as a copy, the foregoing deficiencies may not be a tremendous problem. However, when the imaged substrate is utilized as a printing plate in subsequent conventional offset lithography, the foregoing problems can be detrimental. In this situation, the fixed toner must of course be ink receptive, since such is used as the inked image area. Furthermore, the image must be of high density so as to provide high contrast to the copies prepared by lithography.
I have now discovered a simple combination of materials capable of providing a dry magnetic toner powder which is pressure-fixable and which can be utilized as the image areas of a conventional offset lithographic printing plate.